Lithography masks used in the fabrication of semiconductor devices have a topography. The topography changes, alters and/or blocks radiation passing therethrough in the exposure of a photoresist layer over the semiconductor device layers. Optics are used to focus radiation as it is sent through the mask. While the optics are very precise, the physics of producing a semiconductor device with small dimensions results in discrepancies between the topographies of the mask and the semiconductor device to the extent that modifications to the mask topography will aid in creating the semiconductor device with the desired topography.
Several techniques have been developed that facilitate the fabrication of a semiconductor device with a desired topography. Exemplary techniques include selective attenuation (e.g., using binary photomasks), interference (e.g., using phase shifting masks), reflection (e.g., using extreme ultraviolet reflective optical elements), and beam steering. Sub-resolution assist features (SRAF) have been used to improve performance of masks, such as phase shifting masks. The complex design of modern semiconductor devices which rely on features with increasingly smaller dimensions, however, require improvements in phase shifting masks.